Demand defrost control system

ABSTRACT

A demand defrost system is provided in which a periodic switch device is driven at variable speed in response to relative humidity values ambient to one or more refrigeration units. The periodic switch device initiates defrost cycles in the refrigeration units at variable periodic intervals under the demand constraint of relative humidity. The refrigeration units are controlled by individual constant speed timers which control the maximum period of a given defrost cycle and terminate the cycle early if defrost criterion is satisfied prior to the end of the maximum interval. These timers slave themselves to the periodic switch device to readily receive defrost initiation commands therefrom and include by-pass means to selectively permit real time control of the defrost cycles.

[ Dec. 17, 1974- 1 1 DEMAND DEFROST CONTROL SYSTEM [75] Inventors: Karl J. Schulze-Berge, Manitowoc; John Joseph Allard, Mishicot, both of Wis.

[73] Assignee: AMF Incorporated, White llains,

[22] Filed: Apr. 10, 1973 [21] Appl. No.: 349,690

Primary Examiner-Meyer Perlin Attorney, Agent, or Firm-George W. Price; John H. Gallagher [5 7] ABSTRACT A demand defrost system is provided in which a periodic switch device is driven at variable speed in response to relative humidity values ambient to one or more refrigeration units. The periodic switch device initiates defrost cycles in the refrigeration units at variable periodic intervals under the demand constraint of relative humidity. The refrigeration units are controlled by individual constant speed timers which control the maximum period of a given defrost cycle and terminate the cycle early if defrost criterion is satisfied prior to the end of the maximum interval. These timers slave themselves to the periodic switch device to readily receive defrost initiation commands therefrom and include by-pass means to selectively permit real time control of the defrost cycles.

48 Claims, IO DraWing Figures SHEET 1 0F 6 PATENTEL, cm: 1 7 I974 M Wmm PATENTEB DEC] 7 I974 SEEN 2 BF 6 PATENTE LEE 1 7 I974 FIG.

1 DEMAND DEFROST CONTROL SYSTEM BACKGROUND OF THE INVENTION Defrost controllers for one or a plurality of refrigeration units are known in the art for defrosting at regular timed intervals or on demand based upon the sensing of frost accumulation on the refrigeration coils of the refrigeration unit or units being controlled.

The timed initiation of defrost cycles with the use of thermal tripping to terminate those cycles and a maximum time interval of termination are also known.

Time controlled defrost cycles can be damaging to food and other perishables in the refrigeration units by initiating defrost cycles unnecessarily. Furthermore, such unnecessary initiation of defrost cycles results in increases in cost of power or energy to drive defrost heaters and re-initiate refrigeration cycles unnecessarily.

Prior art demand systems using the sensing of frost build-up as the controlling parameter for initiation of defrost cycles lack versatility in that in the control of large numbers of refrigeration units at common locations, power overloads can readily occur by the concurrent initiation or overlap of a number of defrost cycles requiring power in excess of proper ratedloads for the power systems servicing those units.

The problem is further complicated by the fact that the designs of various refrigeration units result in varying individual defrost cycle requirements, thereby necessitating individual defrost controllers, either time or demand, for each freezer and further, necessitating that for each multiple installation of timed defrost units an empirical timing chart for that installation must be prepared and utilized to set the various individual controllers to preclude undue concurrent initiation and/or overlap of multiple defrost cycles.

OBJECTS OF THE INVENTION tive humidity to vary the rate of initiation of defrost cy-' cles in said units.

Still another object of the present invention is to provide a new and novel demand defrost control system having a variable rate common controller for a plurality of refrigeration units.

Still another object of the present invention is to provide a new and novel demand defrost control system having a variable rate common controller for a plurality of refrigeration units and an individual defrost cycle timing and control means at each said refrigeration unit selectively controlled by said common controller to initiate a defrost cycle therethrough in response to a common ambient characteristic of said units.

Yet another object of the present invention is to provide a new and novel demand defrost control system for a plurality of refrigeration units which substantially precludes concurrent initiation and undue overlap of individual defrost cycles.

These and other objects of the present invention will become more fully apparent with reference to the following specification and drawings which relate to a preferred embodiment of the invention.

SUMMARY OF THE INVENTION A central controller is provided comprising a multicircuit cam switch means driven by a variable speed motor having its speed proportional to relative humidity. Each circuit cam controls the initiation of the defrost cycle of a separate refrigeration unit, each of the latter having a defrost control timer which is started by the respective cam switch.

Each defrost control timer is capable of terminating the initiated defrost cycle after the expiration of a fixed interval or upon the achievement of a given temperature or pressure limit as is known in the refrigeration art.

Each cam of the multi-circuit cam switch is programmed separately for a given refrigeration unit by placing an appropriate number of trippers on the cam. Although these cams are being driven at variable speed (as a function of relative humidity) and not as a function of time of day, the relative time of defrost initiation for each refrigeration units can be staggered or offset from that of the others by staggering the positions of the various cam trippers on the individual cams.

The trippers constrain the actuation of the corresponding defrost timers at the refrigeration units which timers initiate the defrost cycle for that unit. While the defrost cycle may terminate in response to sensed temperature or pressure conditions, the defrost timer will continue to run for a fixed interval to slave itself into a ready position for receipt of the next initiate defrost cycle command from the corresponding cam in the multi-circuit controller.

Thus, the variable speed multi-circuit controller determines the time between defrost cycle initiations and the localized defrost timers determine the length of the respective defrost cycles.

By-pass switch means are provided at the local defrost timers to preclude the effect of the multi-circuit timer and permit a defrost by time of day operating mode at any given refrigeration unit.

In the drawings:

The objects of the invention will become more fully apparent by reference tothe following description of the drawings wherein like numerals refer to like parts and wherein:

FIGS. 1 and 1A, taken together, comprise is a schematic diagram of a demand defrost control system of the present invention controlling a group of refrigeration units;

FIG. 2 is a side elevational view of a variable speed control module of the present invention illustrating the pivot lever in the up position just prior to actuation thereof;

FIG. 3 is a side elevational view of the control module of FIG. 2 with the pivot lever in the down position just prior to deactivation thereof;

FIG. 4 is a partial view of the control module of FIG.

2 illustrating the ready position immediately proceed THE DEMAND DEFROST SYSTEM Referring in detail to the drawings and with particular reference to FIGS. 1 and 1A, the demand defrost system 10 of the present invention is shown for controlling fourrefrigeration units RUI, RUZ, RU3 and RU4 as an example of a multi-circuit application thereof.

Assuming a common situs for the refrigeration units RUl 5 RU4, such that substantially uniform conditions of relative humidity exist for the said units, a humidity sensor HS is shown as the input constraint for a variable speed drive means VSD (to be more fully described hereinafter) which drives a variable speed motor VSM at speeds proportional to the relative humidity of the situs of the refrigeration units RUI RU4 as sensed by the humidity sensor HS.

The variable speed motor VSM drives a multi-circuit controller MCC having control cam means MCCI M CC4 thereon for controlling, respectively, the initiation of defrost cycles for the refrigeration units RUI RU4.

The variable speed drive means VSD is connected across alternating current leads AC! and AC2 which are common to all the refrigeration units RUI RU4.

The refrigeration unit RUI has defrost and refrigeration controls typical of the other units RUZ RU4 and all like elements of the several units RUI RU4 bear like designations with the appropriate suffices l, 2, 3 and 4 designating the respective units RUI RU4 to which they belong.

The refrigeration unit RUI includes a defrost initiate switch SAI, a by-pass mode selection switch SBPI and a slaving switch 881 all connected in parallel from the alternating current power lead AC1 to one side of the defrost timer motor TMI. The opposite side of the timer motor TM] is connected to the other power lead AC2.

A compressor control switch SCI is connected from the power lead AC1 through a cold control thermostat CC I or the like and an overload protection device P1 to one side of a compressor drive motor CMI, the other side of the latter being connected to the other power lead AC2.

A defrost heater control switch SDI is connected from the power lead AC1 to one side of a defrost heater DH 1, the other side of the latter being connected to the other power lead AC2.

Connected across the defrost heater DH! is a temperature or pressure responsive defrost termination switch TPI in series with a defrost termination solenoid winding TSI. Thus, the switch TPI and solenoid winding TSI are in series with the defrost heater control switch SDI. I

The first control cam MCCI on the multi-circuit controller MCC drives the normally open defrost cycle initiation switch SAl into closed position one or more times per given revolution by the control connection El schematically shown in broken lines.

The closing of the defrost initiate switch SAI completes a circuit through the timing motor TMI for a time at least sufficient for the said timing motor TMI to close its sustaining and slave switch SBI via the control connection Fl shown schematically in broken lines, which switch 831 maintains the said timing motor running for a predetermined timed interval. The timing motor TMl is a synchronous motor locked into the line frequency across the power leads AC1 and AC2.

Substantially simultaneously with the closing of the sustaining and slave switch 881, the timing motor TMI, through control connection GI, shown schematically in broken lines, opens the normally closed compressor control switch SCI and closes the normally open defrost heater control switch SDI. This initiates the defrost cycle in the first refrigeration unit RUI by completing a circuit through the defrost heater DHI.

While the timing motor TMl continues to run, the energization of the defrost heater DHI causes the temperature to rise in the refrigeration unit RUI until such time as the normally open temperature or pressure sensitive switch TPI senses that sufficient defrost has been achieved.

At this point in time the switch TPl closes, completing a circuit through the defrost heater switch SDI, and the termination solenoid winding TSI, thereby energizing the latter.

Energization of the termination solenoid coil TSI actuates the control connection HI, schematically shown in broken lines, to open the defrost heater switch SDI and close the compressor control switch SCI, thereby terminating the defrost cycle and initiating the cooling cycle in the refrigeration unit RUI.

In the meantime, because of the closure of the sustaining and slave switch SBI, the timing motor contin ues to run for a predetermined maximum interval until, via the control connection Fl, it re-opens the slave switch 831. This breaks the circuit to the timing motor TMI and causes it to stop.

The control connection Fl by means of cams or other suitable timing means known in the art, is adjusted to open the sustaining and slave switch 881 and thus stop the timing motor TMI in a ready position for substantially immediate resumption of the defrost cycle and reclosure of the sustaining and slave switch 881 upon the next closure of the defrost initiate switch SAl. Closure of the latter, as previously described, completes a circuit through the timing motor TMl around the then open sustaining and slave switch 581, causing the timing motor TMI to start and reclose the said sustaining and slave switch SBl via the control connection F1 for another period of time until the ready position previously described has been achieved and the sustaining and slave switch SBI has been reopened.

A second predetermined maximum time interval for the defrost cycle is effected by the control connection GI, the latter acting to maintain the defrost heater switch SDI closed and the compressor control switch SCI open for that second interval in the event that the temperature or pressure sensitive switch TPI does not cause the termination solenoid TS] and its control connection HI to override the control connection G1 and cause the said switches SCI and- SDI to reverse the initial state constrained upon them by the control connection G1. I

In further explanation of the operation, the control connection F1 is shown as including a timing cam FIA having a short duration rise and dwell portion FIB which cooperates with a cam follower FIC engaging the sustaining and slave switch SBI. Accordingly, when the timer motor TMl is energized by the closure of the defrost initiate switch SA! in the automatic mode (demand defrost) or by the by-pass switch SBPI if the motor TMI is to be driven in a timed defrost mode, the initial rotation of the cam FIA causes the follower FIC to drop off the rise FIB and close the sustaining and slave switch SBI.

When in the automatic mode, the defrost initiate switch SAI is opened after a short period of time by the multi-circuit controller MCC (control cam MCCl) and thus, as the control cam FIA completes a full cycle via the connection FI and timer motor TMI, the rise FIB engages the follower FIC and opens the sustaining and slave switch SIB, thereby stopping the timer motor TMI in a ready position to resume another defrost cycle as constrained by the defrost initiate switch SAl in the demand mode or the by-pass switch SBPI if a timed defrost mode is desired.

The balance of the defrost timing control at the refrigeration unit RU], namely, the timer motor TMl, control connection Gl, compressor control switch SCI, defrost heater switch SDI control linkage HI and defrost termination solenoid TSI are all embodied in commercially available defrost timers such as the 8140 Series of Paragon Electric Company, Inc., Two Rivers, Wis., described in Bulletin No. 374; 68, of that company. Such timers provide control interconnections for the cold control switch CCI, overload protection device OPI, compressor motor CMI, temperature or pressure switch TPI and defrost heater DHl as described with reference to FIG. I.

As previously stated for other elements in FIG. 1 the control cam FlA, rise and dwell FIB and follower FIC are included in the refrigeration units RU2 RU4 by proper suffix correlation and function in the same manner described in connection with the refrigeration unit RU I.

THE MULTI-CIRCUIT CONTROLLER A multi-circuit controller MCC suitable for the present invention comprises a plurality of timing modules MCCI MCCN, each module including in combination: a rotary primary timing dial; a rotary secondary timing dial; drive shaft means coupled to said secondary dial for rotating the same; reduction gear train means for coupling said secondary dial to said primary dial; actuator means selectively disposed on each of said dials for initiating and terminating a timing cycle; a spring biased pivot lever having cam surfaces thereon for engaging said actuator means at selected time intervals; and switch means adapted for engagement with said pivot lever means in response to actuation thereof by said actuator means, said switch means initiating or terminating a timing cycle.

Each of said timing modules MCCI MCCN includes a support plate which may be removably snapped into place in a modular support frame. The respective modules are disposed side by side in said support frame and the drive shafts of the respective modules are releasably coupled together. The timing modules are interchangeable so that when one inoperative module is removed for repair, a replacement can be immediately installed.

Means are provided for programming the multiple module unit MCC so that only one timing module MCCl MCCN will initiate a timing cycle at any given time. This prevents the occurrence of an overload in the refrigeration defrost circuits being controlled.

The timer of the present invention is provided with a drive means comprising the variable speed electric motor VSM disposed on multiple timer module frame.

The timing modules MCCI MCCN described herein each initiate a defrost cycle in one freezer of a bank of freezers at predetermined relative times and include means for terminating this defrost initiation after a predetermined rotary displacement of the timing dials.

Referring in detail to FIGS. 2 to 7 there is shown a typical timing module MCCl including a main support plate 26. Support plate 26 is provided with notches 26A, 26B, 26C for receiving module support bars 40A, 40B and 40C, respectively, of a support frame to be more fully described hereinafter with respect to FIG. 8. A spring biased lever 28 is pivotally mounted on support plate 26 and is provided with a notch 28A for receiving support bar 40A and latching plate 26 thereto. Lever 28 is spring biased toward the position shown so that when module MCC! is placed on the respective support bars, bar 40A will snap into the opening defined by notches 26A and 28A and will be retained therein by spring biased lever 28.

The operative components of the timing module MCCI generally include a secondary timing dial 12 mounted on a main drive shaft 34 journaled in plate 26, a primary timing dial 14 mounted on shaft 148 journaled in plate 26, a micro-switch SAI for initiating a defrost cycle, and a spring biased pivot lever 16 having cam surfaces thereon for constraining lever 16 to actuate microswitch SAI in response to the engagement of the cam surfaces with actuator means disposed on the respective timing dials.

Timing dials l2 and 14 are coupled together by a reduction gear train including: pinion gear 36 affixed to drive shaft 34; a first reduction gear 30 coupled to pinion gear 36 and having a pinion gear 30A affixed thereto; a second reduction gear 32 coupled to pinion gear 30A; and pinion gear 38 affixed to shaft 143 of primary timing dial 14. The variable speed motor means VSM is coupled to drive shaft 34 and energization of variable speed motor means VSM facilitates the simultaneous rotation of both secondary dial I2 and primary dial 14. The reduction gear train is so designed that secondary dial 12 makes 12 revolutions for each revolution of primary dial 1 4.

Primary dial I4 is provided with slots 14A for selectively receiving substantially U-shaped loading actuator means 24. For reasons that will become more fully apparent hereinafter with reference to FIG. 8, slots 14A may be spaced at positions corresponding to twelve equal intervals. Of course slots 14A can be spaced at any other equally spaced intervals as required for various application. U-shaped actuator means 24 may be selectively inserted into any desired slot or slots 14A. The operation of loading actuator 24 will be described hereinafter.

Secondary dial 12 mounted on drive shaft 34 includes a defrost-initiate-on actuating arm 48 and an end-defrost initiate actuating arm 42. Arms 48 and 42 are provided with actuating pins 48A and 42A, respectively, for engaging cam surfaces 16B and 16D of pivot lever 16 in a manner to be more fully described hereinafter.

As shown most clearly in FIG. 6 defrost-initiate-on actuating arm 48 is pivotally mounted about a pin 48C on the surface of dial 12 for movement in slot 46 defining a sector of the surface of circular dial 12. A torsion spring 48B is provided on pin 48C and has a linear portion which extends into abutment with a'post 50. The arrangement of spring 483 on pin 48C is such that arm 48 is normally biased against post 50 but when placed under stress, arm 48 will move across slot 46 and engage boss 46A. With arm 48 held against boss 46A actuating pin 48A is rigidly supported for engagement with cam surface 16E of pivot lever 16. As dial l2 rotates in a clock-wise direction pin 48A will clear cam surface 16B and snap back against post 50. This snap action of arm 48prevents pin 48A from hanging up on cam surface 1615.

As shown most clearly in FIG. 6 end-defrost-initiate actuating arm 42 is mounted for rotation on shaft 34 with respect to the surface of dial l2. Dial 12 is provided with an annular ring of grooves 44 spaced at one minute increments. Arm 42 has a spring detent portion 423 formed therein which may be selectively snapped into any selected groove 44 by rotating arm 42 about shaft 34. By these means substantially any initiatedefrost time period may be selected in the range between zero and 2 hours, i.e., the duration of closure of the defrost initiate switch SAl. In actual practice this time is preselected to insure an initiation signal of sufficient length (i.e. interval of closure of SA! SA4) to permit the slave switches SBl $84 of the corresponding refrigeration unit RUl RU4 to close under all conditions of relative humidity and thus dial speeds produced by the latter.

Actuating arm 42 is further provided with an actuating pin 42A which is operatively associated with pivot lever 16 to engage cam surface 16E, as dial 12 is rotated clock-wise to constrain lever 16 out of engagement with the defrost initiate micro-switch SA! to thereby terminate the defrost-initiate cycle.

Pivot lever 16 is mounted for rotation on a pivot pin 16F, which is journaled in support plate 26. A hole 26D is provided in plate 26 and is operatively associated with a pin 16H which extends from lever 16 into hole 26D to limit the range of rotation of lever 16 about pivot pin 16F. Stated another way the edges of hole 26D function as stop means for lever 16 as it is rotated in either direction. Lever 16 is provided at the bottom thereof with a shoulder portion 16C which rests in a groove in an over center C-spring 18. C-spring 18 is so disposed that it will snap over center as lever 16 is rotated in a clock-wise direction to hold end portion 168 of lever 16 in engagement with actuator buttons SAIA of micro-switch SAl.

Lever 16 is provided with a plurality of cam surfaces 16A, I6D and 16E which are adapted to engage actuator means on dials l2 and I4. Cam surface 16A is adapted to engage actuators 24 as dial 14 is rotated in a counter-clockwise direction. The engagement of cam surface 16A with actuators 24 constrains lever 16 to rotate clockwise to a ready position, as shown in FIG. 4, which flexes C-spring 18 to approximately the center position thereof. Cam surface 165 is adapted to engage actuating pin 48A as dial 12 is rotated clockwise, which flexes C-spring 18 over center and pivots lever 16 in a clock-wise direction to thereby move end portion 163 of lever 16 into engagement with buttons SAlA of defrost-initiate micro-switch SAl. Cam surface 16D is adapted to engage actuating pin 42A as dial 12 rotates in a clockwise direction. The engagement of pin 42A with cam surface 16D constrains lever 16 to rotate counter-clockwise, whereby C-spring l8 snaps back over center and 16B rises from engagement with buttons SAlA of the defrost-initiate micro-switch SAl.

Referring to FIG. 5 there isshown a top plan view of dials l2 and 14 illustrating calibration marks and indicia thereon. Dial 12 is provided with marks and indicia comprising a scale 12A thereon the increments of which correspond to settings of the defrost-initiate cycle on the dial. Arm 42 has a V-shaped notch 42C therein to facilitate reading of the indicia corresponding to each selected setting. Dial 14 is provided with two scales 14C and 14D. Scale 14C is provided adjacent notches 14A to designate the time of day on a 24 hour clock to which each notch 14A corresponds. The corresponding calibration marks on scales 14C and 14D are approximately 210 out of phase to allow for the distance between the top of dial l4 and the portion of dial 14 adjacent cam surface 16A.

If driven by a synchronous motor at constant timed speed instead of by the variable speed drive motor VSM and the foregoing time intervals will be in absolute units of time. As previously disclosed, however,

these intervals are relative intervals to the total intervals on the several dials. The time of day indicia on the dials provides a familiar relative reference for the user in setting up the multi-circuit controller MCC to effect a staggering of defrost cycle initiation in a multiplicity of refrigeration units RUl RUN.

Referring in detail to FIG. 8 there is shown a modular system including a plurality of timing modules MCCl MCCN mounted in a common support frame 54 including side plates 54A and 543 connected together by a plurality of transverse support bars 40A, 40B and 40C.

The variable speed drive motor VSM is secured to the outside of a vertical plate 62 by screws or any other suitable means and is connected to a main drive shaft 56 journaled in plate 62. A gear train 58 is coupled to drive shaft 56 and is housed in a space provided between side plate 54A and vertical plate 62. Gear train 58 has a stub 60 journaled in plate 54A and is provided with a slot 60A therein for receiving a stub 34A on the drive shaft 34 of a first timing module MCCl.

An auxiliary variable speed drive motor VSM may be mounted on plate 548 and may be energized simul taneously with variable speed motor VSM to drive the timing modules MCCl MCCN. Therefore, if either motor VSM or VSM should fail, the other motor will continue to drive timing modules MCCl MCCN without interruption. in the alternative variable speed motor VSM only could be initially energized and control means could be provided for energizing the auxiliary motor VSM upon failure of motor VSM.

As illustrated in FIG. 8 a plurality of timing modules MCCl MCC4 are disposed side by side in support frame 54 on support bars 40A, 40B and 40C. The drive shafts 34 of each respective timing module MCCll MCC4 are coupled together by the respective stubs 34A and slots 348. The modular construction of FIG. 8 has the advantage that if one module must be removed for repair and no replacement is immediately available, then the modules can be shifted to fill the void without substantially interrupting the defrostinitiate timing cycles of the other modules.

In a preferred embodiment the spaced slots 14A of primary timing dial 14 of the adjacent timing modules are staggered so that no two timing modules can be set to trigger a defrost-initiate cycle at the same time. -As shown in FIG. 8 from left to right, four timing modules are illustrated having slots provided which are staggered by 7% of angular rotation from each other. The timing modules illustrated-in FIG. 8 have U-shaped operators 24 inserted therein to facilitate the initiation of defrost cycles that are one-half hour apart if the primary dial I4 is turning one revolution per 24 hours representing the worse anticipated case of 60 percent relative humidity or l hours apart if said primary dial I4 is turning one revolution per 72 hours representing the lowest anticipated relative humidity'of percent.

The purpose for staggering the slots 14A, as shown in FIG. 8, is to prevent more than one defrost heater DHI DH4 from turning on at any given time. If more than one heater could possibly turn on at a given time a fuze could blow out or a circuit breaker might be tripped, causing a power failure to the freezers RUI RU4 and resulting in the spoiling of frozen foods disposed in the freezers RUl RU4. In a large supermarket this could be a very costly accident. With the timing module system of FIG. 8, it is impossible for an operator to accidently set any two timing modules for the same defrost-initiate time, since no two timing modules are provided with spaced slots 14A at the same time settings. Therefore, the possibility of a power failure caused by more than one defrost heater DHI DH4 turning on at a given time is virtually eliminated.

The timing dials may of course be calibrated in smaller or larger increments and the slots 14A staggered in other relationships without departing from the spirit and scope of this invention. It should also be understood that although four timing modules MCCl MCC4 are illustrated in FIG. 8, any number of timing modules MCCl MCCN may be used commensurate with the number of freezers RUl RUN which need to be controlled.

For example, the support frames 54 of FIG. 8 could be constructed to hold four to eight individual timing modules and as many as three to eight frames 54 could be ganged together for controlling a plurality of freezers in a large supermarket. In this type of application it may be possible with slots staggered at 7 angular intervals for more than one timer to turn on simultaneously. However, the number of timing cycles which may be simultaneously initiated is kept to a minimum. This can be minimized even further by staggering spaced slots 14A at smaller intervals, such as three and three quarter degree intervals or any other intervals which will give the desired result for the particular number of load devices being controlled.

The timing modules MCCl MCC4 may easily be inserted into support frame 54 by sliding slots 26C of plate 26 onto support bar 40C and pivoting plate 26 until slot 26B receives support bar 40B and spring biased lever 28 is snapped into place around support bar 40A. Modules MCCl MCC4 may be removed by lifting lever 28 to the position shown in dotted lines in FIGS. 2 and 3 and by pivoting support plate 26 in the opposite direction. This modular construction enables an operator to quickly replace or add a timing module to the system in a minimum amount of time with minimum effort.

DESCRIPTION OF OPERATION The relative point in time at which a defrost cycle is to begin is chosen by inserting one or more U-shaped operators 24 into selected slots 14A in primary timing dial 14. The period of duration of the defrost-initiate cycle is then chosen by positioning end-defrost-initiate actuating arm 42 at a selected position on the secondary dial 12. Any defrost-initiate period (i.e., period of closure of the defrost-initiate switches SAl 8A4), within the range of the secondary dial 12 may be selected.

A timing cycle then begins by energizing the variable speed drive motor VSM which causes drive shaft 34 coupled to secondary timing dial 12 to rotate in a clockwise direction. The rotary force of timing dial 12 is transferred to primary timing dial 14 by way of the reduction gear train including gears 36, 30, 30A, 32, 38 and causes timing dial 14 to rotate in a counterclockwise direction. Dial 14 rotates at one-twelfth the speed of dial l2. Stated another way dial 12 makes 12 complete revolutions for every single revolution of the primary dial 14.

As timing dial 14 rotates operator 24 will come into contact with cam surface 16A of pivot lever 16, as shown in FIG. 4. This position may be designated the ready position, because as operator 24 pushes against cam surface 16A, lever 16 begins to pivot in a clockwise direction until C-spring 18 is displaced approximately to the center position thereof where it is ready to be pushed over center by actuating pin 48A. At substantially the same time actuating pin 48A of spring biased pivot arm 48 comes into contact with cam surface 16E on bottom surface of the tab extending from the rear of pivot lever 16.

As dial 12 continues to rotate arm 48 traverses slot 46 in the surface of dial 12 until it rigidly abuts boss 46A, as shown in FIG. 2. When this occurs pin 48A pulls upwardly on cam surface 16E causing spring 18 to snap over center, thereby rotating lever 16 clockwise against push buttons SAlA of the defrost-initiate switch SAl as illustrated in FIG. 3. Micro-Switch SAI is connected in circuit with the timer motor TMI as previously described with reference to FIG. I.

As spring 18 snaps over center and pivot lever 16 moves into engagement with defrost-initiate microswitch SAl, spring biased arm 48 snaps clear of pivot lever 16 to a position against post 50.

Lever 16 will remain in the position shown in FIG. 3 until actuating pin 42A on end-defrost-initiate actuating arm 42 rotates into engagement with the upper surface 16D of the tab extending from the rear of lever 16. As pin 42A engages surface 16D it constrains lever 16 to rotate counter clockwise, thereby lifting lever 16 from engagement with defrost-initiate switch 8A1 and terminating the defrost-initiate cycle. The timing dials l2 and 14 continue to rotate until the next cycle is initiated in a like manner.

If it is desired to control the defrost cycles of a plurality of freezers such as in a supermarket or warehouse, a plurality of timing modules MCCl MCC4 are provided in a common support frame 54, as shown in FIG. 8, and the respective microswitches SAl SA4 are connected to the separate defrost circuits (at RUl RU4). All of the timing modules are driven by the common variable speed motor VSM.

In programming a multi-circuit defrost timer of the type shown in FIG. 8 it is important to assure that only one defrost cycle is initiated at any given time. Therefore, as stated with respect to the description of FIG. 8, the slots 14A of the adjacent timing modules MCC! MCC4 are staggered to prevent an erroneous setting of the modules by an operator. With the slots provided as shown in FIG. 8, it is impossible to set any two timing modules for the initiation of a defrost cycle at the same time.

Referring to the operation of FIG. 8 an operator may select the defrost cycles for the respective modules and freezers being controlled by selectively inserting operators 24 into slots 14A on primary timing dials 14. The duration of each defrost-initiate cycle is then set by positioning arm 42. The arms 42 of the respective timing modules may be set at selected positions to effect defrost-initiate cycles of compatible length with the ener-.

gization of the respective timing motors TMl TM4 as controlled by the sustaining and slave switches SBl 884. As drive shaft 56 rotates each respective module goes through a timing cycle as described above.

THE VARIABLE SPEED DRIVE Referring to FIG. 9 there is illustrated a variable speed drive circuit generally indicated VSD for a variable speed A.C. hysteresis motor VSM, which is adapted to drive multi-circuit timer MCC. The function of control circuit VSB is to vary the speed of motor VSM as a function of changes in ambient relative humidity sensed by humidity sensor I-IS.

A DC. bias voltage is provided for variable speed drive VSD through A.C. supply lines AC1, AC2, and full wave rectifier FWR to DC. supply lines DCI and DC2.

Motor VSM is provided with a bifilar wound coil Bl having two parallel branches each consisting of one half the turns of coil Bl for driving motor VSM. DC power is supplied to coil B1 from line DCI. The parallel branches of coil Bl are connected in circuit with the collectors of transistors Q3 and Q4, respectively.

Transistors Q3 and Q4 are connected for alternate conduction in a conventional bistable multivibrator configuration wherein the collector-of transistor Q3 is coupled to the base of transistor Q4 through resistor R6 and the collector of transistor 04 is coupled to the base of transistor Q3 through resistor R7. The bases of transistors Q3 and Q4 are coupled to a common emitter junction at DC2 through resistors R8 and R9, respectively.

Because of the coil inductance of B1 in the collector circuits of transistors Q3 and Q4, the multivibrator will free run as an RL multivibrator to drive motor VSM. The multivibrator can be triggered by momentarily connecting both bases of transistors Q3 and Q4 to their emitters through diodes D1, D2 and buffer amplifier transistor Q2.

The multivibrator circuit will free run as a function of the back EMF of the transformer action of the motor coil B1 which acts as a transformer. This transformer action only permits a gradual build-up of motor coil current compared to the apparent impedance of the coil itself.

The transformer action of the coil Bl acts as a memory for the multivibrator to enable the multivibrator to recall which of the transistors Q3 and Q4 was on and which transistor was off. As motor drive current flows through one branch of the coil B1 it acts as a transformer primary and the other branch acts as a transformer secondary. The secondary furnishes an exponential base drive current to the on transistor. During the latter part of the exponential current drive pulse, the following value of di/dt causes the on transistor to come out of saturation and start towards cutoff. The collector current of the on transistor changes at a slower pace due to the motor inductance. The motor current is then diverted from the on transistor collector to the off transistor base, thus turning it on. This transistor is regenerative and ends with the formerly on transistor cut off and the off transistor saturated.

The multivibrator including transistors Q3 and Q4 can be driven synchronously by pulses furnished through transistor Q2 by a unijunction relaxation oscillator O1 to be described hereinafter. The pulses are supplied by the unijunction at rate which varies with the ambient relative humidity sensed by humidity sensor HS.

Transistor O2 is provided with the base thereof connected in circuit with base B2 of a unijunction transistor Q1. Transistor Ql is connected in a relaxation oscillator configuration including: a variable resistor HS connected between DCl and the emitter or gate E of unijunction Q1; a timing capacitor C1 connected between the emitter E and DC2; a biasing resistor R1 connected between DCl and base B1; and biasing resistor R2 connected between base BZ and DC2. Humidity sensor resistor HS and capacitor C1 determine the RC time constant and frequency of the oscillator. Therefore, the frequency of the unijunction oscillator will vary with changes in ambient relative humidity. Base B2 of unijunction transistor Q1 is connected to the base of transistor Q2 to turn Q2 on and off at a rate determined by the frequency of the oscillator.

Although transistors Q2, Q3 and Q4 are disclosed as NPN transistors it should be understood that PNP transistors could be used in the alternative by reversing the polarity of the DC. supply without departing from the spirit and scope of this invention.

In operation motor VSM and rnulti-circuit timer MCC are driven at a rate determined by the alternate conduction of transistors Q3 and Q4 which are driven in synchronism with the frequency of pulses supplied by unijunction transistor 01 through transistor Q2 and diodes D1, D2. Since the frequency of the unijunction transistor oscillator Q1 varies with changes in relative humidity sensed by resistor HS, the speed of motor VSM and the drive shaft of multi-circuit timer MCC will also vary with changes in relative humidity. Therefore, the rate of occurrence of defrost cycles can be varied as a function of changes in the relative humidity of the air surrounding the freezers to be controlled.

If actual time of day on the multicircuit controller MCC and its modules MCCl MCC4 etc., is chosen to represent the worst conditions of relative humidity and temperature ambient to refrigeration units RUl e RU4 etc., to be controlled by the demand defrost system 10, then better conditions of relative humidity and temperature will result in a slower drive speed of the variable speed motor VSM as constrained by the humidity sensor HS and the variable speed drive circuit VSD. This will result in a lesser number of defrost cycle initiations over a given 24 hour period and a more efficient operation of the refrigeration units RUl RU4.

Accordingly, if the primary dial 14 on each control module MCCl MCC4 is provided with six trippers 24, appropriately staggered between modules to preclude simultaneous defrost initiation as previously described, the minimum interval between defrost cycle initiations would be 4 hours.

Accordingly, under these assumptions, the timer motors TMl TM4 in the refrigeration units RUl RU4, by appropriate drive ratios to the control cams FlA F 4A, can be constrained to maximum energization cycles of 4 hours (i.e., one revolution of the said cams FlA F4A for 4 hours subsequent to closure of the defrost initiate switches SAl SA4 by the pivot levers 16 of the respective control modules MCCl MCC4), which interval will place the timer motors TMl TM4 and their respective sustaining and slave switches SBl SB4 in the ready position for all anticipated defrost initiation constraints from the multi-circuit controller MCC.

Thus, upon the receipt of a defrost initiation command, i.e. closure of a defrost-initiate switch SAl 8A4, the respectively associated timer motor TMl TM4 will run for 4 hours, then stop and wait for the next defrost initiation signal from the appropriate con trol module MCCl MCC4.

The actual defrost cycle in a given refrigeration unit RUl RU4 is not 4 hours, but is-controlled either by the termination solenoids TSl T84 and the control connections H1 H4 (and their related temperature or pressure sensitive switches TPl TF4) or by the maximum defrost interval set on the defrost control switch SDl SD4 via the respective control connections G1 G4 as is known in the art for commercially available defrost control timers.

Thus, the variable speed motor VSM and the multicircuit controller MCC controls the time between the initiation of the respective defrost cycles and the constant speed time motors TM 1 TM4 determine the length of the defrost period for the refrigeration units RUl RU4 subject to earlier termination by the termination solenoids TSl T84.

If a non-demand (regularly timed) defrost mode is desired for any given one of the refrigeration units RUl RU4 is desired, then the appropriate by-pass switch SBPl SBP4 is shifted from the open (AUTO) position to the closed or manual (MAN) position to remove the constraint of the multi-circuit controller MCC from that particular refrigeration unit RUl RU4 by shunting the defrost-initiate switches SAl 8A4 and the sustaining and slave switches SBl $84. This permits the appropriate one of the timer motors TMl TM4 to run continuously, synchronized to actual time of day and control the defrost cycles of its associated refrigeration unit RU! RU4 according to actual time between defrost cycles.

What is claimed is:

1. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambient parameter, comprising:

transducer means sensing said ambient parameter and providing a variable speed drive output as a function thereof;

control means driven by said drive output;

constant speed drive means;

first means responsive to said control means energizing said constant speed drive means for a first predetermined interval;

second means responsive to said constant speed drive means maintaining the latter energized for a second predetermined interval; and

third means responsive to said constant speed drive means actuating said defrost means and precluding actuation of said refrigeration means for a third predetermined interval.

2. The invention defined in claim 1, wherein said ambient parameter comprises relative humidity.

3. The invention defined in claim 1, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

4. The invention defined in claim 3, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

5. The invention defined in claim 1, wherein said control means comprises periodic switch actuating means;

said first means comprises first switch means;

said second means comprises second switch means in parallel with said first switch means and cam means contrlling same, said cam means being driven by said constant speed drive means; and

said third means comprises third and fourth complimentary switch means energizing and deenergizing, respectively, said defrost means and said refrigeration means and vice-versa.

6. The invention defined in claim 5, wherein said ambient parameter comprises relative humidity.

7. The invention defined in claim 5, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said vari able speed drive output.

8. The invention defined in claim 7, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

9. The invention defined in claim ll, wherein said defrost controlling means further includes:

override means responsive to an internal parameter 'of a said refrigeration unit constraining said third 10. The invention defined in claim 9, wherein said 7 ambient parameter comprises relative humidity.

11. The invention defined in claim 9, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

12. The invention defined in claim 11, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

13. The invention defined in claim 9, wherein said control means comprises periodic switch actuating means;

said first means comprises first switch means;

said second means comprises second switch means in parallel with said first switch means and cam means controlling same, said cam means being driven by said constant speed drive means; and

said third means comprises third and fourth complimentary switch means energizing and deenergizing, respectively, said defrost means andsaid refrigeration means and vice-versa.

14. The invention defined in claim 13, wherein said ambient parameter comprises relative humidity.

15. The invention defined in claim 13, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output'comprising said variable speed drive output.

16. The invention defined in claim 15, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

17. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambient parameter, comprising:

variable speed motor means having means for providing a defrost initiation constraint at periodic intervals varied as a function of the value of said ambient parameter;

constant speed motor means responsive to said defrost initiation constraint initiating actuation of said defrost means to the exclusion of said refriger- I ation means for a predetermined maximum interval; and said defrost means including override means for deactuating and actuating said refrigeration means prior to the expiration of said predetermined interval in response to an internal parameter of said refrigeration unit. 18. The invention defined in claim 17, wherein said constant speed means includes sustaining and slave means indexing same to a ready condition for response to said defrost initiation constraint prior to the end of the current one of said periodic intervals.

19. The invention defined in claim 17, wherein said ambient parameter comprises relative humidity.

20. The invention defined in claim 18, wherein said ambient parameter comprises relative humidity.

21. Means controlling defrost initiation cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising:

variable speed motor means including means for providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter;

constant speed motor means in each of said units responsive to respective defrost initiation constraints from said variable speed means initiating actuation of said defrost means to the exclusion of said refrigeration means in each of said units for a predetermined maximum interval; and

override means in each of said units for deactuating said defrost means and actuating said refrigeration means in a said unit prior to the expiration of said predetermined interval in response to an internal parameter of the corresponding said unit.

22. The invention defined in claim 21, wherein each said constant speed means includes sustaining and slave means indexing same to a ready condition for response to the corresponding ones of said defrost initiation constraints prior to the end of each corresponding current one of said periodic intervals.

23. The invention defined in claim 21, wherein said ambient parameter comprises relative humidity.

24. The invention defined in claim 22, wherein said ambient parameter comprises relative humidity.

25. Means controlling defrost cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising:

transducer means common to said units sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means common to said units driven by said drive output and providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter;

constant speed drive means in each of said units;

first means in each of said units, responsive to correlated ones of said periodic defrost initiation constraints, energizing the correlated one of said con stant speed drive means for a first predetermined interval;

second means in each of said units responsive to the correlated one of said constant speed drive means maintaining the latter energized for a second predetermined interval; and

third means in each of said units responsive to the correlated one of said constant speed drive means actuating the correlated one of said defrost means and precluding actuation of the correlated one of said refrigeration means for a third predetermined interval.

26. The invention defined in claim 25, wherein said ambient parameter comprises relative humidity.

27. The invention defined in claim 25, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

28. The invention defined in claim 27, wherein said ambient parameter .comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

29. The invention defined in claim 25, wherein said control means comprises a plurality of periodic switch actuating means corresponding one to each said refrigeration unit;

said first means comprises first switch means;

said second means comprises second switch means in parallel with said first switch means and cam means controlling same, said cam means being driven by the correlated one of said constant speed drive means; and

said third means comprises third and fourth complimentary switch means energizing and deenergizing, respectively, the correlated ones of said defrost and refrigeration means and vice-versa.

30. The invention defined in claim 29, wherein said ambient parameter comprises relative humidity.

31. The invention defined in claim 29, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

32. The invention defined in claim 31, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

33. The invention defined in claim 25, wherein said defrost controlling means further includes, in each said unit:

override means responsive to an internal parameter of the correlated one of said units constraining the correlated one of said third means to deactuate and actuate, respectively, the correlated ones of said defrost and refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameters.

34. The invention defined in claim 33, wherein said ambient parameter comprises relative humidity.

35. The invention defined in claim 33, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

36. The invention defined in claim 35, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

37. The invention defined in claim 33, wherein said control means comprises a plurality of periodic switch actuating means corresponding one to each said refrigeration unit;

said first means comprises first switch means;

said second means comprises second switch means in parallel with said first switch means and cam means cntrolling same, said cam means being driven by the correlated one of said constant speed drive means; and

said third means comprises third and fourth complimentary switch means energizing and deenergizing, respectively, the correlated ones of said defrost and refrigeration means and vice-versa.

38. The invention defined in claim 37, wherein said ambient parameter comprises relative humidity.

39. The invention defined in claim 38, wherein said transducer means includes ambient parameter sensing means, variable frequency'oscillator means providing a variable frequency output proportional to said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.

40. The invention defined in claim 39, wherein said ambient parameter comprises relative humidity;

and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.

41. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambient parameter, comprising:

transducer means sensing said ambient parameter and providing a variable speed drive output as a function thereof;

control means driven by said drive output;

constant speed drive means;

first means responsive to said control means energizing said constant speed drive means for a first predetermined interval;

second means responsive to said constant speed drive means maintaining the latter energized for a second predetermined interval; and

third means responsive to said constant speed drive means actuating said defrost means and precluding actuation of said refrigeration means for a third predetermined interval; and

by-pass means selectively disabling said first and second means, constraining said constant speed means to drive continuously and control said defrost and refrigeration means as a function of time and said third means.

42. The invention defined in claim 41, wherein said straint of an ambient parameter common to said units, comprising: f

transducer means common to said units sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means common to said units driven by said drive output and providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; constant speed drive means in each of said units; first means in each of said units, responsive to correlated ones of said periodic defrost initiation constraints, energizing the correlated one of said constant speed drive means for a first predetermined interval; second means in each of said units responsive to the correlated one of said constant speed drive means maintaining the latter energized for a second predetermined interval; third means in each of said units responsive to the correlated one of said constant speed drive means actuating the correlated one of said defrost means and precluding actuation of the correlated one of said refrigeration means for a third predetermined interval and by-pass means in each of said units selectively disabling correlated ones of said first and second means, constraining the correlated one of said constant speed means to drive continuously and control the correlated ones of said defrost and refrigeration means as a function of time and said third means. 44. The invention defined in claim 43, wherein said defrost controlling means further includes, in each said 20 unit:

override means responsive to an internal parameter of the correlated one of said units constraining the correlated one of said third means to deactuate and actuate, respectively, the correlated ones of said defrost and refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameters.

45. Means controlling defrost initiation cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising:

variable speed means providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simulta neous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; and

constant speed means in each of said units responsive to respective defrost initiation constraints from said variable speed means initiating actuation of said defrost means to the exclusion of said refrigeration means in each of said units for a predetermined maximum interval.

46. The invention defined in claim 45, wherein said constant speed means includes sustaining and slave means indexing same to a ready condition for response to said defrost initiation constraint prior to the end of the current one of said periodic intervals.

47. The invention defined in claim 46, wherein said ambient parameter comprises relative humidity.

48. The invention defined in claim 45, wherein said ambient parameter comprises relative humidity. 

1. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambient parameter, comprising: transducer means sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means driven by said drive output; constant speed drive means; first means responsive to said control means energizing said constant speed drive means for a first predetermined interval; second means responsive to said constant speed drive means maintaining the latter energized for a second predetermined interval; and third means responsive to said constant speed drive means actuating said defrost means and precluding actuation of said refrigeration means for a third predetermined interval.
 2. The invention defined in claim 1, wherein said ambient parameter comprises relative humidity.
 3. The invention defined in claim 1, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said aMbient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 4. The invention defined in claim 3, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 5. The invention defined in claim 1, wherein said control means comprises periodic switch actuating means; said first means comprises first switch means; said second means comprises second switch means in parallel with said first switch means and cam means contrlling same, said cam means being driven by said constant speed drive means; and said third means comprises third and fourth complimentary switch means energizing and de-energizing, respectively, said defrost means and said refrigeration means and vice-versa.
 6. The invention defined in claim 5, wherein said ambient parameter comprises relative humidity.
 7. The invention defined in claim 5, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 8. The invention defined in claim 7, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 9. The invention defined in claim 1, wherein said defrost controlling means further includes: override means responsive to an internal parameter of a said refrigeration unit constraining said third means to deactuate said defrost means and actuate said refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameter.
 10. The invention defined in claim 9, wherein said ambient parameter comprises relative humidity.
 11. The invention defined in claim 9, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 12. The invention defined in claim 11, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 13. The invention defined in claim 9, wherein said control means comprises periodic switch actuating means; said first means comprises first switch means; said second means comprises second switch means in parallel with said first switch means and cam means controlling same, said cam means being driven by said constant speed drive means; and said third means comprises third and fourth complimentary switch means energizing and de-energizing, respectively, said defrost means and said refrigeration means and vice-versa.
 14. The invention defined in claim 13, wherein said ambient parameter comprises relative humidity.
 15. The invention defined in claim 13, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 16. The invention defined in claim 15, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 17. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambieNt parameter, comprising: variable speed motor means having means for providing a defrost initiation constraint at periodic intervals varied as a function of the value of said ambient parameter; constant speed motor means responsive to said defrost initiation constraint initiating actuation of said defrost means to the exclusion of said refrigeration means for a predetermined maximum interval; and said defrost means including override means for deactuating and actuating said refrigeration means prior to the expiration of said predetermined interval in response to an internal parameter of said refrigeration unit.
 18. The invention defined in claim 17, wherein said constant speed means includes sustaining and slave means indexing same to a ready condition for response to said defrost initiation constraint prior to the end of the current one of said periodic intervals.
 19. The invention defined in claim 17, wherein said ambient parameter comprises relative humidity.
 20. The invention defined in claim 18, wherein said ambient parameter comprises relative humidity.
 21. Means controlling defrost initiation cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising: variable speed motor means including means for providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; constant speed motor means in each of said units responsive to respective defrost initiation constraints from said variable speed means initiating actuation of said defrost means to the exclusion of said refrigeration means in each of said units for a predetermined maximum interval; and override means in each of said units for deactuating said defrost means and actuating said refrigeration means in a said unit prior to the expiration of said predetermined interval in response to an internal parameter of the corresponding said unit.
 22. The invention defined in claim 21, wherein each said constant speed means includes sustaining and slave means indexing same to a ready condition for response to the corresponding ones of said defrost initiation constraints prior to the end of each corresponding current one of said periodic intervals.
 23. The invention defined in claim 21, wherein said ambient parameter comprises relative humidity.
 24. The invention defined in claim 22, wherein said ambient parameter comprises relative humidity.
 25. Means controlling defrost cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising: transducer means common to said units sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means common to said units driven by said drive output and providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; constant speed drive means in each of said units; first means in each of said units, responsive to correlated ones of said periodic defrost initiation constraints, energizing the correlated one of said constant speed drive means for a first predetermined interval; second means in each of said units responsive to the correlated one of said constant speed drive means maintaining the latter energized for a second predetermined interval; and third means in each of said units responsive to the correlated one of said constant speed drive means actuating the correLated one of said defrost means and precluding actuation of the correlated one of said refrigeration means for a third predetermined interval.
 26. The invention defined in claim 25, wherein said ambient parameter comprises relative humidity.
 27. The invention defined in claim 25, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 28. The invention defined in claim 27, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 29. The invention defined in claim 25, wherein said control means comprises a plurality of periodic switch actuating means corresponding one to each said refrigeration unit; said first means comprises first switch means; said second means comprises second switch means in parallel with said first switch means and cam means controlling same, said cam means being driven by the correlated one of said constant speed drive means; and said third means comprises third and fourth complimentary switch means energizing and de-energizing, respectively, the correlated ones of said defrost and refrigeration means and vice-versa.
 30. The invention defined in claim 29, wherein said ambient parameter comprises relative humidity.
 31. The invention defined in claim 29, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 32. The invention defined in claim 31, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 33. The invention defined in claim 25, wherein said defrost controlling means further includes, in each said unit: override means responsive to an internal parameter of the correlated one of said units constraining the correlated one of said third means to deactuate and actuate, respectively, the correlated ones of said defrost and refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameters.
 34. The invention defined in claim 33, wherein said ambient parameter comprises relative humidity.
 35. The invention defined in claim 33, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output as a function of said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 36. The invention defined in claim 35, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 37. The invention defined in claim 33, wherein said control means comprises a plurality of periodic switch actuating means corresponding one to each said refrigeration unit; said first means comprises first switch means; said second means comprises second switch means in parallel with said first switch means and cam means cntrolling same, said cam means being driven by the correlated one of said constant speed drive means; and said third means comprises third and fourth complimentary switch means energizing and de-energizing, respectively, the correlated ones of said defrost and refrigeration means and vice-versa.
 38. The invention defined in claim 37, wherein said ambient parameter comprises relative Humidity.
 39. The invention defined in claim 38, wherein said transducer means includes ambient parameter sensing means, variable frequency oscillator means providing a variable frequency output proportional to said ambient parameter, and variable speed motor means responsive to said variable frequency output comprising said variable speed drive output.
 40. The invention defined in claim 39, wherein said ambient parameter comprises relative humidity; and said ambient parameter sensing means comprises variable impedance means responsive to changes in relative humidity.
 41. Means controlling defrost cycles of at least one refrigeration unit having defrost and refrigeration means in response to the demand constraint of an ambient parameter, comprising: transducer means sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means driven by said drive output; constant speed drive means; first means responsive to said control means energizing said constant speed drive means for a first predetermined interval; second means responsive to said constant speed drive means maintaining the latter energized for a second predetermined interval; and third means responsive to said constant speed drive means actuating said defrost means and precluding actuation of said refrigeration means for a third predetermined interval; and by-pass means selectively disabling said first and second means, constraining said constant speed means to drive continuously and control said defrost and refrigeration means as a function of time and said third means.
 42. The invention defined in claim 41, wherein said defrost controlling means further includes: override means responsive to an internal parameter of a said refrigeration unit constraining said third means to deactuate said defrost means and actuate said refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameter.
 43. Means controlling defrost cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising: transducer means common to said units sensing said ambient parameter and providing a variable speed drive output as a function thereof; control means common to said units driven by said drive output and providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; constant speed drive means in each of said units; first means in each of said units, responsive to correlated ones of said periodic defrost initiation constraints, energizing the correlated one of said constant speed drive means for a first predetermined interval; second means in each of said units responsive to the correlated one of said constant speed drive means maintaining the latter energized for a second predetermined interval; third means in each of said units responsive to the correlated one of said constant speed drive means actuating the correlated one of said defrost means and precluding actuation of the correlated one of said refrigeration means for a third predetermined interval; and by-pass means in each of said units selectively disabling correlated ones of said first and second means, constraining the correlated one of said constant speed means to drive continuously and control the correlated ones of said defrost and refrigeration means as a function of time and said third means.
 44. The invention defined in claim 43, wherein said defrost controlling means further includes, in each said unit: override means responsive to an internal parameter of the correlated one of said units constraining the correlated one of said third means to deactuate and actuate, respectively, the correlated ones of said defrost and refrigeration means prior to the expiration of said third predetermined interval in response to a predetermined value of said internal parameters.
 45. Means controlling defrost initiation cycles of a plurality of refrigeration units, each such unit having refrigeration and defrost means, in response to the demand constraint of an ambient parameter common to said units, comprising: variable speed means providing defrost initiation constraints for each of said units at staggered periodic intervals to substantially preclude simultaneous initiation of defrost cycles in two or more of said units, said periodic intervals being varied as a function of the value of said ambient parameter; and constant speed means in each of said units responsive to respective defrost initiation constraints from said variable speed means initiating actuation of said defrost means to the exclusion of said refrigeration means in each of said units for a predetermined maximum interval.
 46. The invention defined in claim 45, wherein said constant speed means includes sustaining and slave means indexing same to a ready condition for response to said defrost initiation constraint prior to the end of the current one of said periodic intervals.
 47. The invention defined in claim 46, wherein said ambient parameter comprises relative humidity.
 48. The invention defined in claim 45, wherein said ambient parameter comprises relative humidity. 